MIG vs TIG vs Friction Stir Welding for Aluminum

Because aluminum alloys are used so widely, their welding processes now include pulsed gas metal arc welding, AC gas tungsten arc welding (TIG), and friction-stir welding, among others.

Aluminum alloys generally have good hot and cold workability and weldability and can be joined by conventional fusion processes. Common options include gas-shielded arc, argon-arc, plasma-arc, resistance, and electron-beam welding. Processes with high thermal power, concentrated energy, and effective shielding are particularly suitable.

Rail-transit equipment is currently welded mainly by automated gas-shielded processes. Advanced methods such as laser-arc hybrid welding remain in process development and limited-area application trials.

MIG Welding

Equipment

A MIG system consists mainly of a power source, controller, wire feeder, welding gun, cable assembly, and ground lead.

Characteristics

MIG offers high productivity, and high-current MIG is especially suitable for thick plate. Distortion is somewhat larger after welding, and quality depends partly on equipment performance. Controlling weld porosity is the central concern. Hydrogen may enter from several sources.

Metal-Transfer Mode

The three transfer modes are short-circuiting, spray, and projected spray. Short-circuiting uses a short arc with a popping sound and shallow penetration. It is used mainly for fillet or butt joints in 1–2 mm walls with low current and 0.8 or 1.0 mm wire. Spray transfer has a longer arc with a whistling sound and produces narrow, deep finger penetration; incomplete fusion at the sides, porosity, and cracks may result. Projected-spray transfer has a slightly longer arc and a mild popping sound. It produces a basin-shaped penetration profile and better mechanical properties than either shallow short-circuiting penetration or finger-shaped spray penetration.

Projected-spray transfer is generally recommended for aluminum MIG welding.

Factors Affecting Quality

Argon is the usual shielding gas; Ar-He mixtures can increase efficiency and reduce porosity. Wire-feed speed, gun angle, travel speed, and nozzle height must be controlled. Use a PTFE or nylon liner rather than a metal spring liner, and do not use a serrated drive roll.

Note: 1.L = first pass; 2.L = second pass; filler wire matches the base metal; shielding gas is Ar; G = back-side pass.

Groove Design

Joint geometry is determined by material thickness and structural design. Aluminum groove angles should be larger than steel groove angles because root defects form readily; the larger opening improves root conditions.

In the flat position, point the gun in the travel direction at 5°–20° from vertical, with the wire biased toward the side that conducts heat away more strongly.

Material thicker than 8 mm requires multiple passes. The root pass is critical because many aluminum-weld defects occur at the root. Make it fast with a small leg size so deposited metal does not cover the root and trap defects.

TIG Welding

Equipment

A TIG system consists mainly of a welding power source, controller, and torch.

Characteristics

TIG is suitable for thin sheet and offers low distortion, low porosity, and high quality for demanding products. Use AC TIG with back-side gas shielding. The negative half-cycle removes the oxide film, while the positive half-cycle reduces tungsten overheating. Typical porosity causes are low-purity argon, dirty wire or grooves, ineffective shielding, unsuitable parameters, excessive electrode extension, and an arc that is too long or unstable.

Operating Technique

The usual technique for a right-handed welder is forehand welding: hold the torch in the right hand, the wire in the left, tilt the torch 20° to the right, and feed the wire toward the pool at 15° to the workpiece.

First establish a weld pool with the arc. Feed the wire into it from beneath the arc using a repeated forward-and-back motion. Do not move the wire too far under the arc or pool temperature will fall. Melt the wire tip into the leading edge of the pool and keep it inside the shielding envelope.

Friction-Stir Welding

Equipment

The process is performed by a rotating tool consisting of a profiled pin, holder, and cylindrical shoulder.

Principle and Characteristics

The rotating tool transfers frictional heat into the joint, bringing the material into a plasticized thermoplastic state as the tool travels along the seam. At the start, the profiled pin rotates rapidly and enters the joint. Friction between the pin and metal forms a thermoplastic layer, while shoulder contact supplies additional heat. Plasticized metal continually moves from the front of the tool to the rear and fills the cavity. The weld zone undergoes compression, frictional heating, plastic deformation, transfer, diffusion, and recrystallization.

Advantages include:

1. Solid-state joining.
2. Little or no distortion.
3. Ability to join difficult materials such as high-strength aluminum, castings, and composites.
4. No shielding gas.
5. Excellent mechanical properties, particularly fatigue performance.
6. No harmful smoke, infrared radiation, or ultraviolet radiation.
7. High travel speed.
8. No fusion-welding defects such as pores and cracks.
9. Normally no pre-weld preparation in the joint area.
10. No columnar grains in the joint; plastic flow refines the grains.

Limitations include:

1. No weld reinforcement; workpiece ends must form a butt joint, and fillet welds cannot be made.
2. Three-dimensional curved surfaces cannot currently be welded.
3. The protruding tool feature can leave an exit hole at the end of the joint.
4. A backing support must resist the frictional force.
5. Joint mismatch and gap require tight control.
6. Production use is currently limited to light metals.